Three groups of young shoots were lit from one side only for two days.
- Group A: shoots untreated
- Group B: tips covered with an opaque (light-proof) cap
- Group C: tips cut off
(a) Shoots in group A grew towards the light. Explain why. [3]
(b) Predict the results for groups B and C. [2]
- (a) auxin is produced in the (shoot) tip(1)
- (a) auxin moves / diffuses to the shaded side; accept more auxin on the shaded side / auxin unevenly distributed (more away from light)(1)
- (a) causes (cell elongation) faster growth on the shaded side, so shoot bends towards light(1)
- (b) B: grows straight up / does not bend(1)
- (b) C: does not bend / no response to light, with no (or very little) growth; accept 'no growth' alone; reject 'bends towards the light'(1)
- Answer: (a) auxin made in tip, moves to shaded side, cells there elongate more so shoot bends (b) B straight up; C no bending, little growth
- Examiner tip: Auxin causes cells on the SHADED side to elongate — saying the light side grows faster is the most common error.